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Brands and Plate Quality.                     281

be well squeezed in both directions when made into plate. Steel
plates are much more reliable now than when first introduced, it
being clearly recognised that a certain amount of strength must
be sacrificed to ductility. They are not, therefore, considerably
stronger than* iron, but much more homogeneous or even in
structure, the particles being so thoroughly re-arranged when in the
molten state. Iron plates, on the other hand, are very various in
quality, even over one plate,1 because of the processes employed
in obtaining fibre. A test strip, either for plate or bar, rarely gives
an exact determination of the 'whole, while* the contrary holds
with steel. Steel plates are termed c mild' because they have little
more carbon than wrought iron plates ; they have some 30 per
cent, more strength than the latter, with one and.a half to twice
the elongation. The price of iron being also greater, it is not
surprising that steel is the only material now used for plate work,
excepting where continuous flame action (as in locomotive fire-
boxes) renders Lowmoor iron or copper preferable. Copper,
though more expensive, and losing its strength somewhat while
hot, is an "exceedingly good conductor of heat, and deteriorates
less under the action of flame, lasting therefore longer than iron,
while being more efficient

Steel, then, in a mild, homogeneous form, is the material now
generally used for all plate and angle work. Both steel and iron
are received from the rolling mill under the following forms:—

Plates         ---------

Angle Bais         L
Tee Bars           T

Channel Bars    n

H (Aitch) Bars            H

Flat and Square Bars •••
Round Bars                 •

Brands, qualities, and sizes of plates.—The qualities
of iron have been mentioned at p. 76, * common' being used for
bridges and girders, and the remainder for boiler work. Mild
Steel occurs in four qualities, thus :—

1.  Ship and bridge quality.      ,

2.  Ordinary boiler quality.

3.  Soft boiler quality.

4.  Superior quality (to resist flaime).